Method of making rigid particle boards or the like

ABSTRACT

A particle web is made by spreading upon a support surface superimposed layers of particles coated with a heat hardenable glue. The first outside layer is spread employing particles which are unheated, a center layer is then spread on the first layer employing particles which have been preheated to a temperature just below the hardening temperature of the glue, and a second outside layer of unheated particles is then spread on the center layer. The web is then promptly pressed between pressing members heated to a temperature above the hardening temperature of the glue to form a rigid member. The result of the preheating of the particles is a more rapid pressing operation and a better outer surface of the resulting pressed member. A small amount of water may be applied to the exposed surfaces of the web just prior to the pressing so that steam generated by contact between the pressing members and the water assists in carrying heat to the center of the web.

Mar h 1972 B. J. CARLSSON 3,649,396

METHOD OF MAKING RIGID PARTICLE BOARDS OR THE LIKE:

Filed Jan. 22, 1970 3 Sheets-Sheet 1 BENGT J. CARLSSON INVENTOR BUCKHORN, BLORE, KLARQUIST & SPARKMAN ATTORNEYS March 14, 1972 B. J. CARLSSON 3,649,396

METHOD OF MAKING RIGID PARTICLE BOARDS OR THE LIKE Filed Jan. 22, 1970 3 Sheets-Sheet 2 Fig.2

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BENGT J. CARLSSON INVENTOR BUCKHORN, BLORE, KLARQU! ST EL SPARKMAN ATTORNEYS mh 1 1972 a. J. CARLSSON METHOD OF MAKING RIGID PARTICLE BOARDS OR THE LIKE 3 Sheets-Sheet 3 Filed Jan.

Fig. 3

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BENGT J. CARLSSON INVEN TOR BUCKHORN, BLORE, KLARQUIST 8. SPARKMAN ATTOR NEYS United States Patent US. Cl. 156-622 4 Claims ABSTRACT OF THE DISCLOSURE A particle web is made by spreading upon a support surface superimposed layers of particles coated with a heat hardenable glue. The first outside layer is spread employing particles which are unheated, a center layer is then spread on the first layer employing particles which have been preheated to a temperature just below the hardening temperature of the glue, and a second outslde layer of unheated particles is then spread on the center layer. The web is then promptly pressed between pressing members heated to a temperature above the hardening temperature of the glue to form a rigid member. The result of the preheating of the particles is a more rapid pressing operation and a better outer surface on the resulting pressed member. A small amount of water may be applied to the exposed surfaces of the web just prior to the pressing so that steam generated by contact between the pressing members and the water assists in carrying heat to the center of the web.

In the manufacture of particle board or similar hot pressed products, there are first prepared finely divided particles, such as wood chips, which are then heat dried to a specific desired moisture content and screened to remove particles of unwanted size. The accepted particles are then collected in a storage bin of sufiicient capacity to equalize normal variations in the supply or interruptions in the particle feed to subsequent processing stages. A flow of particles, maintained as uniform as possible, is passed from the collecting bin to a glue mixer, where a carefully determined quantity of chemicals in aqueous solutions is charged to the flow of particles. The chemicals are charged in such a way that the chemical solution is mainly deposited onto the surface of the particles. Calculated on the dry goods, the particles after being dried may contain, for example 3% water and after being coated with adhesive, for example, 12% water, 9% carbamide glue (urea-formaldehyde resin) and a minor quantity of hardener, the purpose of which is to hasten the hardening of the glue during the following heat pressing operation. In certain instances inhibitors are added instead of hardeners, to delay the hardening of the glue. The particles are passed from the glue mixer to a shaping or forming station, at which they are spread onto a support surface, to form a normally continuously, forwardly growing particle web having a thickness of from 30-150 mm. During the treatment process and transport from the heat drying stage to the web forming stage, the particle temperature falls from, for example, 80 C. in the heat dryer to approximately room temperature at the forming station.

The particle web is most often constructed of three layers, of which the two outer layers are relatively thin and consist of relatively fine particles, thereby imparting to the final product the desired surface properties, while the center layer contains coarser particles, which are cheaper to produce and which impart to the final product other desired characteristics. The two types of particles-- surface layer particles and center layer particlesare nor- 3,649,396 Patented Mar. 14, 1972 'ice mally collected in their respective bins and supplied with different quantities of water and chemicals in their respective glue mixers and are each spread onto the support surface by one particle spreading machine for each surface layer and one or more particle spreading machine for the center layer.

Slabs of desired length are then successively separated from the forwardly expanding web and introduced into a hot press, where they are rapidly compressed to the desired thickness between heated flat surfaces whose temperature is normally maintained at 160-170 C. in multi-story presses and 190-200 C. in single story presses. In order to prevent appreciable hardening of the glue in the surface layers as a result of heating, before the slab has been compressed, rapid presses and a surface pressure of the order of 35 kg./cm. are required. As the heat into the slab during the pressing operation penetrates and the glue hardens, the particles are bound permanently to one another. When this has also occurred in the center portion of the slab, the press can be opened and a finished, rigid board having normally a total moisture content of approximately 8% is removed. Subsequent to completion of the pressing operation, the temperature in the center plane of the particle board is normally between -110 C. and -1 80 C. nearest the surfaces of the board. As a result of the higher degree of heating, the surface layer is almost completely dry while the moisture content of the center layer may be from 10-12% When these differences in moisture content are later equalized after the pressing operation there is a risk that the boards will bend.

The time taken for the glue to harden in the center plane of the particle slabthe so-called net press timeis decisive for the capacity of a plant. By using a press temperature of around 200 C., wherewith pre-hardening of the surface layers can be counteracted by adding inhibitors when applying glue to the surface particles, and by applying to the surfaces of the slabs prior to the pressing operation small quantities of water, in the region of approximately 150 to 200 g./m. which is vaporized during the pressing operation, and by causing heat to be conveyed rapidly in towards the center of the slab, it has been possible to reduce the net press time to about 10 seconds per mm. board thickness.

If the press temperature is further increased, the aforementioned disadvantage regarding the drying of the surface layers also increases. The board surfaces thus take an unwanted structure and color, since the press time is not sufficiently short to prevent the deleterious effect of a high press temperature. Laboratory tests, however, have shown that the press time can be reduced without impairing the quality of the board by dielectric high frequency preheating of the particle slabs immediately prior to their insertion into the press. It has been found that heating of the slabs from 20 C. to 70 C. will shorten the net press time by about 35%. The costs associated with operating and purchasing the equipment for this preheating process are, however, high. For space reasons it is hardly possible to arrange this equipment in existing systems, while in new systems it is diflicult to incorporate into conventional forming and pressing systems. A further disadvantage is that the variations in weight per unit area and in the density of the particle slabs result in uneven heating of the slabs. The aforementioned preheating equipment also gives rise to added disadvantages as a result of its disturbing effect on wireless sound and picture transmission. Finally, it can be mentioned that the temperature in the different layers of the slab cannot be regulated in a desired manner. Although dielectric preheating of particle slabs has been employed industrially for more than 10 years, the aforementioned weaknesses have restricted its use to less than 1% of all particle board production systems in operation during this period.

3 SUMMARY OF THE INVENTION The present invention relates to a method for preheating particle webs, slabs or the like which are intended to be hot-pressed into rigid products, it being possible to practice the method with simple means in systems already in operation and enables preheating to be effected irrespective of the manner in which the particle webs and/or particle slabs are handled and conveyed up to and into the press. The invention permits the manufacture of particle webs having determined different temperatures in different layers of the web, so that the preheat temperature in the surface layers can be maintained lower than in the center layer, whereby higher press temperatures can be used without damaging the surface layers, and also permits the preheat temperature to be independent of any variation in weight per unit area and in denstiy which occur in the web or slab during the shaping thereof. The method of the invention is mainly characterized in that the particles are imparted a temperature corresponding to the desired temperature in the Web or the slab before being distributed by one or more particle spreading machines or corresponding devices positioned at a shaping station to form the particle web or slab.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow sheet of a system of known design for producing particle board,

FIG. 2 is a flow sheet of a similar system adapted for use with the method of the invention,

FIG. 3 is a partial cross section through a particle board or a particle slab and a diagram of the temperature therein in two different instances,

FIGS. 4-5 show in the form of a diagram the temperature obtained in the particle slab at a specific moment during the pressing operation at three different instances of preheating, to illustrate the effect of the different preheating processes.

DESCRIPTION OF PRIOR ART SYSTEMS The known system illustrated diagrammatically in FIG. 1 incorporates a particle station 1, in which particulate material, chips, are produced from Wood or similar vegetabilistic material and dried in a hot gas drier 2 and then divided into different fractions at a screening station 3. Those particles which are considered too coarse are passed from the screening station 3 in the form of a flow 4 back to the particle station 1 for further disintegration. Those particles which are considered too finei.e. dustare removed in another flow 5 and are normally passed to a hot center in the system, where they are burned. The usable particles, center particles and surface particles, are respectively passed to a center particle collecting bin 6 and a surface particle collecting bin 7. A practically constant flow of center particles is removed from the center particle collecting bin 6 and fed through a glue mixer 8, in which the center particles are coated with glue, wherefrom they are passed to one or possibly more center particle spreading machines 9 located at a shaping station generally indicated by the reference numeral 10. A practically constant flow of surface particles is also removed from the surface particle collecting bin 7 and passed through a second glue mixer 11 for surface particles, whereafter the surface particle flow is divided into two flows of essentially equal size, each of said flows being passed to its respective surface particle spreading machines 12 and 13 at the shaping or forming station 10. The particle spreading machines 9, 12, 13 spread the particle flows onto a support surface 14 arranged therebeneath, the surface 14 moving at a uniform spread relative to the spreading machines in the direction shown by the arrow 15. The two surface particle spreading machines 12, 13 are placed in front of and behind the center particle spreading machine 9, as seen in the direction of movement 15. The dispersed particles form a particle web 16, which grows continuously forwardly in the direction of movement 15 of the support surface 14 andwhich coinprises a lower surface particle layer 21, formed by the spreading machine 12, a center particle layer 22, formed by the spreading machine 9, and an upper surface particle layer 23, formed by the spreading machine 13. FIG. 3 illustrates to the left thereof a partial cross section through the formed portion of the particle Web. As shown in FIG. 1, particle slabs 17 of determined lengths are parted from the particle web 16 by means of a saw 18. The particle slabs 17 are then charged to a hot press 19, in which they are pressed to rigid particle boards 20. Small quantities of Water are applied to the top and bottom surfaces of the slabs 17 immediately prior to their insertion into the press.

Subsequent to their leaving the press 19, the pressed particle boards .20 are then normally subjected to varying treatment processes. These processes are not illustrated in the flow sheet since they do not form a part of the invention. The direction in which the material flows through the system is illustrated by the arrows on the lines connecting the different units.

The fiow sheet illustrated in FIG. 1 has been simplified to a certain extent. Normally such a system would also include other units, e.g. wet chip'collecting bins for equalizing daily variations between particle manufactureand particle consumption etc. Such units have been excluded since they are of no importance to the invention. The various units and working processes of the system are in general so well known to one having normal skill in the art that a more detailed description is considered unnecessary.

DESCRIPTION OF PREFERRED EMBODIMENT The system illustrated in FIG. 2 embodies the same units as those of the system illustrated in FIG. 1 and the same references have been used in both figures. 'In the system illustrated in FIG. 2, however, there is arranged between the glue mixer 8 and the spreading machine 9 as a heating means, a center particle heater 32. Furthermore, a heat insulating means 33 is arranged for the spreading machine 9 and for the conveying line between the particle heater 32 and the spreading machine 9, to prevent cooling of the center particles heated in the heater 32, and to retain as far as possible the temperature which was imparted to the particles in the heater when they were spread by the spreading machine 9 onto the support surface 14 and formed the center layer 22 of the particle web 16. The particle heater 32 is, of course, also heat insulated.

It is unimportant to the method of the present invention how the particle heater 32 is designed or how the particles are heated therein. The glue-coated particles, however, prior to being pressed in the hot press 19 must not be subjected to temperatures which cause appreciable hardening of the glue. -In the case of those types of glues used for particle board, hardening takes place slowly or extremely slowly at low temperatures, although the hardening rate increases above a certain temperature limit and accelerates very rapidly with increasing temperatures. Such glues used at present should not be subjected to temperatures higher than about 75 C. prior to pressing, at least not for more than extremely short periods. On the other hand, it is desired to preheat the particles to as close to the critical temperature as possible. Accordingly, the particles should be heated in the heater 32 to a temperature which is as uniform as possible without risk of local overheating. One suitable heating method is to cause steam to condensate onto the particles. The particles are exposed in the heater to a steam-saturated air stream having a temperature of 70-75" C. An increase in the temperature of the particles from 20 C. to 70 C., however, requires condensation of a quantity of water which increases the moisture content of the particles by about 4%. In order that the moisture content of the finished particle slab 17 is not unsuitably high, it may therefore be necessary to reduce the water content of the glue. It is also possible to reduce the quantity of water condensed on the particles by preheating the particles in some other manner before introducing them to the heater 32, and merely effect a final adjustment of the temperature in the heater.

As previously mentioned, FIG. 3 illustrates to the left a partial cross section through the particle web 16. This section is of course also applicable to a cross section through the slab 17. The diagram to the right of the figure shows in principle how the temperature varies over the cross section in two different instances.

As mentioned in the preamble of the description, the particles in the system illustrated in FIG. 1 have a relatively high temperature, e.g. 80 C., when leaving the hot gas drier 2, but that, before the particles reach the spreading machines 9, 12, 13, this temperature falls to approximately ambient temperature. When practising the known method of producing particle web 16, an approximately uniform temperature of say 20 C. is obtained through the entire particle web 16 and slab 17, as is shown by the line 24 in the diagram of FIG. 3.

By, in accordance with the invention, imparting to the center particles an elevated temperature before they are spread by the machine 9 onto the support surface 14 it is theoretically possible, however, to provide a temperature in the particle web =16 and the particle slab 17 according to the curve 25. Owing to unavoidable heat losses behind the particle heater 32, a somewhat lower temperature can be expected in the center layer 22 of the web than the temperature which is imparted to the particles in the particle heater, say for instance 65 C., as shown by curve 25. In reality, heat is of course exchanged between the center layer 22 and the surface layers 21 and 23, this heat exchange already taking place while the particle web 16 is being shaped and which means that the temperature will deviate from the theoretical in a direction towards a temperature distribution such as that shown by the dotted curve 26. The extent to which the temperature deviates from the theoretical temperature distribution becomes greater with the length of time which has passed since the particles were shaped, and is thus somewhat greater in the first shaped leading end of the particle slab 17to the left of FIG. 2.-when this is passed into the press 19. As a result of the poor heat conducting ability of the loosely packed particles, it can be expected, however, that the temperature distribution in the particle web 16 obtained during the shaping process is essentially found remaining in the particle slab 17 so that the temperature of the slab when it is passed into the press 19 and the press begins to close is approximately that shown in curve 26.

When the press 19 is closed and the particle slab 17 begins to be compressed between the hot surfaces of the press, the water applied to the upper and lower surfaces of the slab rapidly evaporates and the hot steam penetrates into the slab. This activity is called in the following steam infusion. The steam, which has a temperature of 100 C. or slightly higher, condensates on the colder particles, thereby heating the same. It is possible for a portion of the steam to reach the center of the slab before the slab is compressed through the passages formed between the particles in the slab, which, at least at the beginning of the compressing operation, are relatively large. This steam infusion thus provides a. rapid heating of the entire particle slab immediately at the beginning of the pressing operation. The amount of water, applied to the surface of the particle slab are normally adjusted so that the steam infusion increases the temperature of the slab by 35 C. to 40 C. if a uniform increase in temperature was obtained. For this purpose a total quantity of water of 1.5 to 2% based on the weight of dry particles, is required. In reality the steam infusion effect naturally provides a high increase in temperature at the outer layers and a smaller increase in the center of the particle slab, so that a particle slab 17 which has not been pre-heated but which upon being inserted into the press 19 has a uniform low temperature completely throughout, as shown by the line 24 in FIG. 3, would have taken a temperature approximately as that shown in the curve 27 in FIG. 4 when the press was closed and the particle slab fully compressed and all water supplied to the surfaces had been evaporated and had penetrated into the slab and condensed there, if it had not been possible in the time to produce an increase in the temperature of the compressed particle slab by conducting heat into the same. Line 24' in FIG. 4 shows the starting temperature in the particle slab.

In the case of a particle slab 17 which when inserted into the press has cold surface layers 21, 23 and a warm center layer 22 as shown by curve 26 in FIG. 3, the cold surface layers will be heated by the steam infusion effect in approximately the same manner as a particle slab which is cold throughout, although even in this case the center layer would be heated by the condensation of the quantity of steam which passes the surface layers without condensing thereon. This means that the average increase in temperature in the center layer as a result of the steam infusion effect is almost equally as great when the center layer is preheated as when it is cold. But since the center layer has already a higher temperature a large portion of the steam is able to penetrate through its outer portions without being condensed and continue into its center portion. The increase in temperature in the center of the center layer by the steam infusion effect can therefore be assumed to be somewhat greater when the center layer is preheated than when it is cold. Curve 28 in FIG. 5 shows approximately the temperature which a particle slab 17 would have been imparted by the effect of steam infusion if the slab had a starting temperature according to curve 26 in FIG. 3 and 26' in FIG. 5 respectively.

If, before the slab is introduced into the press 19, the center layer of the slab 17 can be imparted a temperature which lies sufficiently close to the temperature at which the glue begins to harden rapidly, it is obvious that the temperature in the center of the particle slab can be increased by the infusion of steam above this temperature at the beginning of the pressing operation. Even if hardening of the particle sla'b must be finished later by heat conduction via the layers 21, 23, it is believed that preheating of the center layer 22 enables the press time to be considerably shortened. The press time is conceivably so short that it is also possible to use higher press temperatures than those usual hitherto without there being sufficient time to damage the surfaces of the board and thereby to obtain a further shortening of the press time.

The temperature curves 27, 28 and 30 shown in FIGS. 4 and 5 however, assume that heat transport by steam takes place instantaneously, so that the particle slab is heated completely throughout, right to the center thereof, at the same moment as steam is formed at the outer surfaces. If this were the case it is obvious that the best result of the steam infusion effect would be obtained, i.e. the highest temperature possible in the center of the particle slab without unnecessary overtemperature at its surfaces, if the outer surfaces 21, 23 had the same starting temperature as the center layer 22, since the surfaces remain at the vaporization temperature until all water present thereon has been vaporized. In reality, however, a certain length of time is required for the steam to penetrate to the center of the particle slab, this length of time increasing with the thickness of the slab. All water at the surface is thus able to evaporate and the temperature of the surfaces begins to rise over the vaporization temperature up towards the temperature of the :press plates before penetration of the steam has achieved its maximum effect in the center of the particle slab. The colder the interior of the outer layers at the moment when all the water at the surfaces has vaporized the more quickly the heat is conducted into the outer layers and the lower is the temperature maintained thereby at the actual surface. If the outer layers 21, 23 are thin and the center layer 22 is thick it may be suitable to impart to the outer portions of the center layer a somewhat lower temperature than its center portion. Different temperatures in different portions of the center layer may easily be provided if the center layer is shaped by means of three or more shaping machines 9.

The invention is not restricted to the shown and described use. It is unimportant which type of particle is used and it is also immaterial whether one forms a continuously forwardly growing particle web 16 which is divided into particle slabs 17 of restricted lengths, whether the particle web 16 is pressed directly in accordance with a continuous pressing method or whether, as is often so, particle slabs or the like of restricted length are shaped directly. It is also principally unimportant how the shaping station 10 is arranged and which means are used for shaping, although acceptable heat insulating means should be provided.

Neither is the invention related with the method or the means for imparting to the particles a suitable temperature before they are spread at the shaping station 10 into particle webs, particle slabs or the like, and the means shown in FIG. 2 and the associated description have only been included as an example as how these can be arranged. The particle heater 32 may also be placed before the glue mixer 8 and 11 instead of before and behind the same, wherewith also the glue mixer should be heat insulated and it may also be suitable to preheat the glue. In this case the requirement of imparting to the particles a uniform temperature in the particle heater is reduced, since equalization of the temperatures between particles having varying degrees of heat can be eifected before the glue is applied.

When the construction of the system renders it possible, it is also expedient to retain the heat obtained by the particles when they leave the hot gas drier 2, by suitably heat insulating all the subsequent units and conveying lines as well as the particle spreading machine 9 at the shaping station 10, thereby reducing the heat consumption of the system. In this case, at least the particles collecting bin 6 should also be provided with heating means for maintaining the temperature of the particles stored therein, in the event of lengthy interruptions in operation of the system, and it may also be suitable to provide other portions of the system with heat retaining means.

What is claimed is:

1. In a method of making rigid particle boards or the like in which a particle web is formed having a center layer of particles coated with a heat hardenable glue positioned between outer layers of particles also coated with a heat hardenable glue and said web is pressed between heated pressing members at a temperature causing hardening of said glue; the improvement which comprises: preheating the particles to be employed for forming said center layer to a temperature below said temperature causing hardening of said glue; thereafter forming said web employing said heated particles for said center layer and employing particles at a lower temperature for said outer layers to produce a diiferentially heated web; and pressing said difierentially heated web between heated pressing members to cause hardening of said glue. I y 2. The method of claim 1 in which: the particles employed to form said center layer are preheated to a temperature of approximately C. :3. The process of claim 1 in which: the preheating of said particles employed to formsaid center layer includes heating by exposure to a steam saturated air stream at a temperature of 70 to C. 4. The process of claim 1 in which: an amount of water between approximately 1.5 and2% based on the weight of the dry particles in the web is applied to the exposed surfaces of the web just prior to said pressing of said dilferentially heated web. References Cited UNITED STATES PATENTS 2,890,146 6/1959 Unsworth 156-373 X 3,051,219 8/1962 Kaiser 156-373 X 3,070,838 1/1963 Hostettler l5636 3,246,064 4/ 1966 Moore et al 156-622 X 3,372,217 3/1968 Paerels et al. 156-369 X 3,383,266 5/1968 Helm 1563l3 3,428,505 2/1969 Siempelkamp 156373 X FOREIGN PATENTS 796,636 6/1958 Great Britain. Y

BENJAMIN A. BORCHELT, Primary Examiner H. J. TUDOR, Assistant Examiner In US. Cl. X.R 156322, 369 

